Method of manufacturing a light filament from carbon nanotubes

ABSTRACT

A light filament ( 206 ) formed from carbon nanotubes is characterized by high mechanical strength and durability at elevated temperatures, a high surface area to volume ratio, and high emissivity. Additionally, electrical resistance of the light filament does not increase with increasing temperature as much as electrical resistance of metallic light filaments. Accordingly, power consumption of the light filament is low at incandescent operating temperatures. A method for making a light filament made of carbon nanotubes includes the steps of: forming an array of carbon nanotubes ( 20 ); pulling out carbon nanotube yarn ( 204 ) from the carbon nanotube array; and winding the yarn between two leads ( 30 ) functioning as electrodes to form the light filament.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention generally relates to a light filament and a method formaking the same, and more particularly to a light filament and a methodfor making the same formed of carbon nanotubes. The instant applicationrelates to the copending applications Ser. Nos. 10/334,547 filed Dec.31, 2002 and 10/335,283 filed on Dec. 31, 2002.

2. Description of the Related Art

Electric light filaments are typically made of materials which areeither polycrystalline in nature or which are amorphous, ornoncrystalline, in nature. Such materials become brittle when they aresubjected to high temperatures for prolonged periods.

Polycrystalline materials, which include the majority of commerciallyavailable metallic filaments, are characterized by the presence ofcrystal grain boundaries, dislocations, voids and various othermicrostructural imperfections. These microstructural imperfections leadto grain growth and recrystallization, particularly at elevatedtemperatures, which in turn lead to increased brittleness and diminishedstrength.

Metallic filaments have relatively low electrical resistivity.Therefore, they are often made quite long and are tightly coiled inorder to fit within a light bulb of suitable size. Coiling of a filamentreduces its effective radiating surface area because parts of the coiledfilaments partially block other parts, thereby diminishing the radiativeefficiency of the filament. This results in a coiled filament's higherelectrical power consumption to produce the same amount of radiatingsurface area. A light filament having a higher surface area to volumeratio can provide greater radiative efficiency.

Hence, an improved light filament that overcomes the aforementionedproblems is desired.

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to provide a lightfilament having a high surface area to volume ratio and greatdurability, particularly at elevated temperatures.

Another object of the present invention is to provide a method formaking a light filament having a high surface area to volume ratio andgreat durability, particularly at elevated temperatures.

In order to achieve the first above-mentioned object, a light filamentin accordance with the present invention is formed from carbonnanotubes. The light filament is characterized by high mechanicalstrength and durability at elevated temperatures required to achieveincandescence. In addition, the light filament is characterized by ahigh surface area to volume ratio and high emissivity compared withconventional metallic light filaments. Additionally, electricalresistance of the light filament does not increase with increasingtemperature as much as electrical resistance of metallic lightfilaments. Accordingly, power consumption of the light filament is lowat incandescent operating temperatures.

In order to achieve the second above-mentioned object, a method formaking a light filament in accordance with the present inventioncomprises the steps of: forming an array of carbon nanotubes; pullingout carbon nanotube yarn from the carbon nanotube array; and winding theyarn between two leads to form the light filament.

Other objects, features and advantages of the present invention willbecome apparent from the following detailed description when read inconjunction with the accompanying drawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic side elevation view of an array of carbonnanotubes formed by a method in accordance with the present invention;

FIG. 2 is a schematic isometric view of a procedure for forming carbonnanotube yarn from the array of carbon nanotubes of FIG. 1, inaccordance with the present invention;

FIG. 3 is a schematic side view of a light filament formed by windingthe carbon nanotube yarn of FIG. 2 between two tungsten leads, inaccordance with the present invention; and

FIG. 4 is a current (I) versus voltage (V) graph, showing empirical I-Vcurves obtained for a light filament made by the method of the presentinvention before and after heat treatment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention is further described below with reference to thedrawings, in which like reference numerals are used to designateidentical or corresponding parts.

Referring now to FIG. 3, the present invention provides a light filament206 comprising carbon nanotubes. The light filament 206 is characterizedby high mechanical strength and durability at the elevated temperaturesrequired to achieve incandescence. In addition, the light filament 206,when wound on two tungsten leads 30, is characterized by a high surfacearea to volume ratio and high emissivity compared with conventionalmetallic light filaments. Additionally, electrical resistance of thelight filament 206 does not increase with increasing temperature as muchas electrical resistance of tungsten light filaments. Accordingly, powerconsumption of the light filament 206 is low at incandescent operatingtemperatures.

A method for making the light filament 206 comprises:

Step 1. Referring to FIG. 1, forming a superaligned array of carbonnanotubes, discussed in greater detail below.

Firstly, a substrate 22 is provided. The substrate 22 includes a siliconwafer 222, which is two inches in diameter and 350 μm thick. An 800 nmthick thermal-oxidized layer 224 is deposited on the silicon wafer 222.A surface of the thermal-oxidized layer 224 is flat and smooth, toenable growth of a large-scale array of carbon nanotubes. Then an ironthin film 24 that is 5 nm thick is deposited on the substrate 22 byelectron beam evaporation, and is subsequently annealed in air at300˜400° C. for 10 hours to form a ferrous oxide film. Then the ferrousoxide film is reduced to pure iron by reaction with hydrogen or ammonia,so that the pure iron can be used as a catalyst.

The substrate 22 is then preferably diced into a plurality ofrectangular pieces. Each such piece is put into a quartz boat, which issubsequently inserted into the center of a one-inch quartz tube furnace.The tube furnace is then heated to 650˜700° C. in flowing argon gas.After that, a mixture of 30 sccm (standard cubic centimeter per minute)acetylene and 300 sccm argon gas is introduced into the tube furnace for5˜30 minutes. Acetylene functions as a carbon source gas, and argonfunctions as a protecting gas. The furnace is then cooled down to roomtemperature. Thus, a superaligned carbon nanotube array 20 is formed onthe substrate 22.

Step 2. Referring to FIG. 2, pulling out carbon nanotube yarn 204 fromthe carbon nanotube array 20.

Carbon nanotube bundles 202 of the carbon nanotube array 20 are pulledout by a tool, for example, tweezers. A carbon nanotube bundle 202 isany group of carbon nanotubes formed in a contiguously adjacent group inthe carbon nanotube array 20. As a carbon nanotube bundle 202 is drawnout, it pulls out other carbon nanotube bundles 202 joined end to end atjoint portions 203 thereof by van der Waals attraction therebetween. Asa result, the yarn 204 is formed.

Step 3. Referring to FIG. 3, winding the yarn 204 between two leadsfunctioning as electrodes to form the light filament 206.

Since the yarn 204 is easily broken by strong or uneven forces, the yarn204 is wound carefully between two tungsten leads 30 which are spacedapart by approximately 1 cm. Silver paste 32 is applied on the tungstenleads 30 at positions where the tungsten leads 30 join with the yarn204, to lower resistance between the yarn 204 and the tungsten leads 30.Thus the light filament 206 is formed, which can emit incandescent lightwhen a DC voltage is applied across the tungsten leads 30.

Based on extensive experimentation on the growth mechanisms of carbonnanotubes, the crucial factors for growing a superaligned carbonnanotube array 20 are listed below:

-   -   a. The substrate 22 should be substantially flat and smooth.    -   b. The growth rate should be relatively high.    -   c. The partial pressure of carbon source gas should be        relatively low.

When the substrate 22 is flat and smooth, a higher density carbonnanotube array 20 can be formed. Because the carbon nanotubes are packedclosely together, van der Waals attraction between adjacent carbonnanotubes is strong, which enables the carbon nanotubes to be pulled outfrom the carbon nanotube array 20 to form the yarn 204. Therefore,non-porous silicon wafer or silicon wafer with a thermal-oxidized filmcan be used as the substrate 22.

If factors b and c above are fulfilled, the carbon nanotubes will bewell graphitized, and will have no deposits on their outer surfaces. Asis known in the art, during the growth of carbon nanotubes, amorphouscarbons are simultaneously deposited on outer surfaces of the carbonnanotubes. This gives rise to considerably less van der Waals attractionbetween the carbon nanotubes. The growth rate of the carbon nanotubesneeds to be high, while the deposition rate of amorphous carbons needsto be low. The growth rate of carbon nanotubes is proportional to thedifference between the furnace temperature and the local temperature ofthe catalyst. Generally, the difference in the temperatures iscontrolled to be at least 50° C., in order to enhance the growth rate ofthe carbon nanotubes. The deposition rate of amorphous carbons isproportional to the partial pressure of carbon source gas. In practice,the local temperature of the catalyst can be controlled by adjusting theflow rate of carbon source gas, and the furnace temperature can bedirectly controlled. The partial pressure of carbon source gas can becontrolled by adjusting the ratio of the flow rates of the carbon sourcegas and the protecting gas. Typically, the partial pressure of carbonsource gas is controlled to be not more than 0.2, and preferably notmore than 0.1.

A combined width of the yarn 204 depends on a number of carbon nanotubethreads in the yarn 204. In general, the combined width of the yarn 204can be controlled by a size of the tips of the tool that is used to pullout the yarn 204. The smaller the tips, the thinner the combined widthof the yarn 204. A force required to pull out the yarn 204 togetherdepends on the combined width of the yarn 204. Generally, the greaterthe combined width of the yarn 204, the greater the force required. Acombined length of the yarn 204 depends on an area of the carbonnanotube array 20.

In alternative embodiments of the preferred method, when forming thecarbon nanotube array 20, other gases such as nitrogen or helium can beused as the protecting gas instead of argon gas. Other metals, such ascobalt or nickel, can be used as the catalyst instead of iron. Othercarbon hydrogen compounds, such as methane or ethylene, can be used asthe carbon source gas.

Preferably, the formed light filament 206 is further treated as follows.The light filament 206 mounted on the leads 30 is put into a vacuumsystem, which is evacuated to 5×10⁻³Pa (Pascals). Then a DC voltage isapplied to the light filament 206 across the tungsten leads 30 for afixed period of time so that the light filament 206 emits incandescentlight. After such so-called heat treatment, the light filament 206 isstronger and more elastic. In addition, it has been found that whenhigher DC voltages are used for the heat treatment, electrical currentin the light filament 206 increases proportionately. It has also beenfound that the tensile strength and the conductivity of the lightfilament 206 can be considerably enhanced by such heat treatment.

In particular, a new light filament 206′ (not illustrated) having newproperties can be formed essentially by performing such a heat treatmenton the light filament 206. A different new light filament 206″ (notillustrated) having different properties can be formed by performing asimilar heat treatment on the light filament 206, but using differentparameters of time and voltage applied. By plotting the I-V(current-voltage) curve and measuring the tensile strength of each ofthe light filaments 206, 206′ and 206″, changes produced in the lightfilament 206 by the two different regimes of heat treatment can beinvestigated.

For instance, when a fixed DC voltage of 50V was applied to one lightfilament 206 for 3 hours, and the light filament 206 was then allowed tocool down, it became the light filament 206′. When a fixed DC voltage of70V was applied to another identical light filament 206 for 3 hours, andsaid another light filament was then allowed to cool down, said anotheridentical light filament 206 became the light filament 206″.

Referring to FIG. 4, the I-V curves of the light filaments 206, 206′ and206″ in vacuum are recorded by using Keithley 237, respectively yieldingcurves A, B and C. As seen, there is no substantial difference betweencurves A and B. However, when comparing curves A and C, a significantincrease in current is attained, especially at higher operatingvoltages. In particular, at the operating voltage 70V, the current ofcurve C is about 13% higher than that of curve A. That is, the lightfilament 206″ carries about 13% more current than the light filament 206at this operating voltage.

Tensile breaking strength tests have been conducted on the lightfilaments 206 and 206″. Tensile breaking strength obtained by straingauge measurement on the light filament 206 and 206″ is 1 mN and 6.4 mNrespectively. That is, the tensile breaking strength of the lightfilament 206 is enhanced more than six-fold after heat treatment for 3hours at 70V to form the light filament 206″.

The enhanced conductivity and tensile strength of the light filament206″ indicates that some structural change has occurred in the lightfilament 206 as a result of said heat treatment. During heat treatmentof the light filament 206, the joint portions 203 of the yarn 204provide the highest electrical resistivity in the light filament 206.Accordingly, these joint portions 203 sustain the highest increases intemperature, and the structure of the light filament 206 at these jointportions 203 may be changed significantly.

It will be understood that the particular devices embodying the presentinvention are shown and described by way of illustration only, and notas limiting the invention. The principles and features of the presentinvention may be employed in various and numerous embodiments thereofwithout departing from the scope of the invention.

1. A method for making a light filament, comprising: forming an array ofcarbon nanotubes having a density sufficient to pull out carbon nanotubeyarn therefrom; pulling out carbon nanotube yarn from the carbonnanotube array; and winding the yarn between two leads functioning aselectrodes to form the light filament.
 2. The method as claimed in claim1, wherein the step of forming an array of carbon nanotubes comprises:providing a substrate having a flat, smooth surface; depositing acatalyst film on the substrate; putting the substrate having thecatalyst film thereon into a furnace; introducing protecting gas whileheating the furnace up to 650˜700° C.; introducing a mixture of carbonsource gas and protecting gas at respective fixed flow rates for 5˜30minutes; and cooling the furnace down to room temperature, whereby anarray of carbon nanotubes is formed on the substrate.
 3. The method asclaimed in claim 2, wherein the substrate includes a non-porous siliconwafer having a flat and smooth surface.
 4. The method as claimed inclaim 2, wherein the substrate includes a silicon wafer and athermal-oxidized layer having a flat and smooth surface deposited on thesilicon wafer.
 5. The method as claimed in claim 2, wherein the catalystfilm is an iron, cobalt or nickel film.
 6. The method as claimed inclaim 5, wherein the iron film is deposited on the substrate, and isthen annealed in air at 300˜400° C. for 10 hours to form a ferrous oxidefilm, and subsequently the ferrous oxide film is reduced to pure iron byreducing gas.
 7. The method as claimed in claim 6, wherein the reducinggas is hydrogen or ammonia.
 8. The method as claimed in claim 2, whereinthe protecting gas is argon, helium or nitrogen.
 9. The method asclaimed in claim 2, wherein the carbon source gas is acetylene, methaneor ethylene.
 10. The method as claimed in claim 2, wherein the carbonsource gas is acetylene, and the protecting gas is argon.
 11. The methodas claimed in claim 2, wherein a ratio of the flow rates of carbonsource gas and protecting gas is not more than 0.2.
 12. The method asclaimed in claim 2, wherein a ratio of the flow rates of carbon sourcegas and protecting gas is not more than 0.1.
 13. The method as claimedin claim 1, wherein the step of pulling out carbon nanotube yarn fromthe carbon nanotube array comprises the step of: pulling out carbonnanotube bundles from the carbon nanotube array.
 14. The method asclaimed in claim 1, wherein the leads are tungsten leads.
 15. The methodas claimed in claim 1, further comprising the step of: applying silverpaste on the leads at positions where the leads join with the yarn, tolower resistance between the yarn and the leads.
 16. The method asclaimed in claim 1, further comprising the steps of: putting the lightfilament into a vacuum system; evacuating the vacuum system to at least5×10⁻³Pa; and applying a DC voltage to the light filament across theleads for at least 3 hours.
 17. The method as claimed in claim 16,further comprising the step of: cooling down the light filament.
 18. Themethod as claimed in claim 16, wherein the DC voltage is 70V.
 19. Themethod as claimed in claim 1, wherein the carbon nanotube yarncomprising a plurality of carbon nanotube bundles which are joined endto end by van der Waals attractive force.
 20. A method for making alight filament, comprising: forming an array of carbon nanotubes;pulling out carbon nanotube yarn from the carbon nanotube array; windingthe yarn between two leads functioning as electrodes to form the lightfilament, and applying silver paste on the leads at positions where theleads join with the yarn, to lower resistance between the yarn and theleads.
 21. A method for making a light filament, comprising: forming anarray of carbon nanotubes; pulling Out carbon nanotube yarn from thecarbon nanotube array; winding the yarn between two leads functioning aselectrodes to form the light filament; putting the light filament into avacuum system; evacuating the vacuum system to at least 5×10⁻³Pa; andapplying a DC voltage to the light filament across the leads for atleast 3 hours.